1. Field of the Invention
This invention relates to a compatible AM-FM frequency discriminator with which it is possible to derive not only an FM demodulated output but also an AFC (Automatic Frequency Control) output for FM signals and an AFC output for AM signals from an FM demodulator employing a quadrature detecting circuit.
2. Description of the Prior Art
An example of a quadrature detecting circuit (hereinafter referred to as the Q detecting circuit) is shown in FIG. 1. An FM intermediate frequency input is applied to balanced differential amplifiers Q.sub.1 and Q.sub.2 in the Q detecting circuit 1 surrounded by the one-dot chain line. FM waves displaced in phase are produced with phase shifters 2 and 3. A demodulated output is obtained from the product of the quadrature-phased and in phase FM waves, and an AFC output can also be obtained. In this instance, the phase shifter 2 is formed with an LC resonant circuit and is tuned to an FM intermediate frequency f.sub.0 to provide abrupt phase variations at the adjacent frequencies, as indicated in terms of reactance variations in FIG. 2A. The phase shifter 3 is formed, for example, with an inductor L to uniformly provide a 90.degree. phase shift in the vicinity of the FM intermediate frequency f.sub.0, as shown in FIG. 2B. With the relative phase difference between the both phase shifters, such a discriminator output as depicted in FIG. 2C is obtained to achieve frequency discrimination. In this case, the phase shifter 3 may be formed with a capacitor C in place of the inductor L but since this phase shifter is also used to provide a DC bias, the inductance L is employed in the present example. The use of the capacitor C will reverse the polarity of the discriminator. The discriminator output is usually employed not only as an FM demodulated output but also as an AFC voltage.
Automatic frequency control which is widely employed in ordinary FM receivers, can also be used in AM receivers. Especially, in the so-called electronic tuning receiver using a semiconductor variable reactance element or the like as a tuner, both AM and FM waves are subjected to automatic frequency control in many cases. In the prior art, as illustrated in FIG. 3, FM and AM systems are provided separately of each other. In the FM system, an FM tuner 11, an IF amplifier and limiter 12, and a frequency discriminator 13 make up a demodulator to derive therefrom a voice output, and the output from the frequency discriminator 13 is fed back to the FM tuner 11 through an AFC circuit 14. In the AM system, an AM tuner 15, an IF amplifier 16 and a detector 17 constitute a demodulator to derive therefrom a voice output, and the output from the IF amplifier 16 is fed back to the AM tuner 15 through a limiter 18, a frequency discriminator 19 and an AFC circuit 20. As described above, the prior art requires special provision of a limiter and a frequency discriminator for the AM waves and, even if they are partly used in common to the FM and AM waves, the circuit structure used is complicated and is difficult to adjust.
In the prior art circuit of FIG. 4, the limiter, the frequency discriminator and the AFC circuit are used in common with the FM and AM waves so as to overcome the aforesaid defects. Namely, the limiter 18, the frequency discriminator 19 and the AFC circuit 20 in FIG. 3 are left out and the branched output from the IF amplifier 16 on the AM side is applied to an IF amplifier and limiter 12' supplied with the output from the FM tuner 11. The output from the IF amplifier and limiter 12' is applied to a compatible AM-FM frequency discriminator 20', the output from which is fed back to the FM and AM tuners 11 and 15 through an AFC circuit 14'. As illustrated, the compatible AM-FM frequency discriminator 20' is designed so that a phase shifter group 22, shown in a broken-line block, is composed of a resonant circuit 22.sub.1 tuned to the AM intermediate frequency and a 90.degree. phase shift circuit 22.sub.2 for the AM waves. Similarly, phase shifter group 23 shown in a broken-line block, is composed of a resonant circuit 23.sub.1 tuned to the FM intermediate frequency and a 90.degree. phase shift circuit 23.sub.2 for the FM waves. Switches are used for connecting the desired phase shifter group 24.sub.1 and 24.sub.2 to Q detecting circuit 21. The above circuit construction is simple, as compared with that of FIG. 3, but involves the use of switches and has the disadvantage of increasing stray capacitance contributed by wiring, the switches, etc., which lowers the accuracy of the phase shifters.